In order to accelerate the spread of solid oxide fuel cell systems (hereinafter, SOFC systems) as stationary power generating units, it is necessary that the utilization of solid oxide fuel cell systems provide a greater advantage than the utilization of conventional grid power supplied from a large-scale power station via a power grid. Therefore, development has been conducted with an objective of realizing high power generation efficiency, longer life, or lower costs of SOFC systems.
For example, in an SOFC system using a hydrocarbon as a raw material gas, steam reforming using steam is utilized in order to reform the raw material gas. Steam reforming (SR) is a reforming method with the highest efficiency, and realizes a long system life of tens of thousands of hours. Therefore, steam reforming is the most preferred reforming method in order to achieve the aforementioned objective.
In an SOFC system, similar to PEFC (Polymer Electrolyte Fuel Cell), PAFC (Phosphoric Acid Fuel Cell), etc., energy necessary for performing the steam reforming is obtained from combustion heat, which is generated as a result of combusting an exhaust hydrogen gas discharged from the fuel cell (hereinafter, the exhaust hydrogen gas is referred to as an anode off gas). A particularly characteristic feature of such a high-temperature fuel cell as a solid oxide fuel cell (hereinafter, SOFC) is that the cell operating temperature of the high-temperature fuel cell (about 700 to 1000° C.) is higher than a temperature necessary for the steam reforming (about 600 to 700° C.).
In an SOFC system, even exhaust heat generated from an exothermic cell reaction can be used as the energy necessary for the steam reforming, and a kind of energy regeneration circuit can be formed between the fuel cell and a reformer. Therefore, the energy efficiency of such SOFC systems is higher than that of other fuel cell systems such as PEFC and PAFC. That is, SOFC systems have a feature of being excellent in terms of power generation efficiency.
In particular, in order to realize more excellent power generation efficiency, SOFC systems adopt a heat insulating structure that is formed by integrating an SOFC, a reformer, a combustor, and the like together. Specifically, SOFC systems include a hot module (a casing part). The hot module houses, at least, an SOFC, a reformer, a combustor, and the like, which are covered with a heat insulating material.
In order to perform the above-described steam reforming, the reformer is packed with a catalyst (a reforming catalyst). It is known that if the catalyst is poisoned by sulfur components contained in the raw material gas, it causes degradation in catalytic activity. For this reason, it is necessary to desulfurize the raw material gas before the raw material is supplied to the reformer.
One example of a method of desulfurizing the raw material gas is a hydrodesulfurization method in which: sulfur components contained in a fuel gas are reduced by hydrogen, so that hydrogen sulfide is generated; and thereafter, the hydrogen sulfide is adsorbed, and thus the fuel gas is desulfurized. The hydrodesulfurization method has excellent features of being effective against a wide range of sulfur components and being applicable to even a high sulfur concentration. For example, Patent Literature 1 proposes a SOFC system of a hot module type, which adopts such a hydrodesulfurization method.